Flashover Arc Research Articles

Influence of Sheath Radial Crack on Flashover Arc and Leakage Current of Roof Silicon Rubber Insulator for High-Speed Train

In this paper, three types of insulators with no crack, short crack, and long crack are taken as the research objects to evaluate the influence of the penetrating cracks on the 1 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">st</sup> sheath of the roof silicone rubber composite insulator of high-speed train on the flashover characteristics. The continuous voltage rising method is used to obtain the influence characteristics of the crack on the development process of the flashover arc. The constant voltage withstand method is adopted to compare the leakage current when the contamination degree is 0.1 mg/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The test results demonstrate that the crack has no significant effect on the flashover voltage and flashover path, and the flashover arc can burn the edge of the crack. The peak leakage current of the three different tests ranges from 24 mA to 30 mA. Additionally, the development of leakage current can be divided into four stages: rising, continuous violence, falling, and oscillating. Moreover, the sequence of the apparent leakage current of the insulator is long crack insulator, short crack insulator, and the one with no crack. The acceleration of the rising stage of the leakage current increases with the increase in the crack length. Meanwhile, the sum of the time of the rising stage, continuous stage, and falling stage of the insulator with long crack is the smallest. The crack will distort the distribution of current density and the electric field intensity in the vicinity. The results can provide references for evaluating the influence of radial cracks at the edge of the sheath on the flashover characteristics and optimizing the design of the roof insulator sheath structure.

AC flashover characteristics and arc development process of glaze ice-covered insulators in natural environment

Ice accretion on the insulators can reduces the creepage distance to a certain extent as low as the dry arc distance, which may cause flashover trip and subsequent blackout. Extensive research on insulator icing has been carried out in a controlled-climate room. Due to great differences in environmental parameters between laboratory simulation and field tests, icing characteristics and discharge development of ice-covered insulator obtained in the laboratory cannot veritably reflect natural icing law and arc propagation process. Consequently, in the present study the glaze-icing on energized insulators was investigated on site, and the mechanism of icicle growth on the insulator energized with different voltage levels was revealed. Meanwhile, AC flashover performances and arc discharge development of ice-coated insulators for the two test conditions were compared and discussed. Results show that for a weak electric field, icicle firstly grows vertically, and then bends to the axis of the insulator when its length reaches to a certain level. Nevertheless, icicle develops towards the axis at the beginning under a high electric field. AC flashover voltage of ice-accreted insulators increases exponentially with an increasing applied voltage, while the relation of flashover voltage and ice weight fits well with a negative power function. Icing flashover voltage of the insulators obtained in natural environment is 14%～21% higher than that in the chamber. Compared with the flashover in the laboratory, during the discharge of the insulators naturally covered with ice, arc extinction-reignition and floating phenomena are more significant, and the ice-melting is more severe.

DC performance of non-uniformly transversal polluted glass insulation under parallel electrical discharges’ effect

The aim of this paper is to quantify the evolution’s effect of parallel electrical discharges on the performance of a plane insulation placed simultaneously under conditions of a non-uniform transversal distribution of pollution and positive DC voltage. The results emanating from this experimental study revealed a maximum increase about 18% of flashover voltage of insulation’s non-uniform transversal distribution of the insulation compared to that obtained under uniform pollution distribution. The existence of a limit value of the electrical conductivities’ ratio of the lightly and the heavily contaminated bands, characterized by an early development of the electrical flashover arc of the insulation in the heavily polluted band, was demonstrated in this investigation. Also, a direct link has been established between the flashover frequency of the insulation’s highly contaminated band, under the effect of a non-uniform transversal distribution of pollution and its electrical resistance. Similarly, an approximation linking the insulation’s length to the effective width of an electrical arc in its last phase of development and consequently the number of electrical arcs that can develop in the two differently polluted bands just before the insulation’s flashover was done in this study.

Influence of snow accretion on arc flashover gradient for various types of insulators

Atmospheric snow accretion along the insulators can eventually bridge adjacent shed spacing, which pose severe risks to the security of power networks. Aiming to investigate the influence of snow accumulation on the electrical performance of various types of insulators, the flashover tests of snow-accreted insulators were conducted. The effects of various factors on the electrical characteristics of insulators were analysed based on the experimental data. Results show that AC arc flashover gradient of the insulator covered with nonuniform snow accumulation is lower than that of the uniform snow-covered insulator. The former is as low as 22.5% lower than the latter. Moreover, the AC arc flashover gradient of the porcelain and glass insulators is 0.27–6.27% higher than that of the DC. For a silicone rubber insulator, this gap is greater than 20%. The flashover gradient of the glass insulator is the minimum at the snow thickness of 10 mm. The flashover process of snow-covered insulator is neither similar to that of the contaminated insulator nor different from that of the iced insulator but a combination of the two. The presented work helps to better understand the flashover mechanism of snow-covered insulators, and take efforts to mitigate its effects.

Comparison of AC Flashover Performance of Snow-Accreted Insulators Under Natural and Artificial Simulation Environments

The research on snow-accreted insulators have been performed in an artificial climate chamber. However, the differences in the distribution, density and compactness of snow accretion under natural and artificial simulation environments cause differences in the flashover performance of snow-covered insulators. In the present work, a series of flashover tests were conducted on snow-coated glass and silicone rubber insulators in the climate chamber and at Xuefeng Mountain Natural Icing Test Base. The discrepancies in the electrical characteristics and flashover process of snow-covered insulators under two test environments were compared and analyzed. Results show that the arc flashover gradient of snow accreted insulator at high altitude site is higher than that in the laboratory because of the differences of the environmental parameters. Affected by the adiabatic effect and capillary action of snow layer, within the range of the study the flashover gradient of insulators covered with snow increases with the increasing in the snow thickness. The calculation of various forces applied on arcs during its propagation demonstrates that when the arc current is less than 0.2 A, the local arcs develop along the snow surface of the insulators under the action of an electrostatic force. As the current increases beyond 0.2 A, the thermal buoyancy is predominant, thereby causing arcs to levitate from the insulator surface.

Distribution of polymer surface charge under DC voltag and its influence on surface flashover characteristics

This study investigates the characteristics of polymer surface charge and their influence on DC surface flashover, which is significant to the study of surface discharge theories and the insulation design of high-voltage direct current equipment. Silicone rubber (SiR), epoxy resin (EP), and polymethyl methacrylate (PMMA), were selected as experimental materials in this paper. Using the active capacitance probe method, the charge accumulation, and dissipation characteristics of the polymers were studied under needle-plane electrode corona discharge. In cases of no surface charge accumulation, DC flashover tests were conducted in multiple electric fields. Furthermore, the influence of surface charge on flashover performance was quantitatively evaluated by means of optical and electrical tests as well as mathematical analysis. The results indicate that the surface potential of EP and PMMA increases more prominently as the applied voltage increases, but SiR is the least to do so. On the other hand, the surface potential of SiR and PMMA decays faster, with EP as the slowest of all. After the polymer accumulates surface charges, the flashover voltage, the luminance of the flashover arc channel, and the fractal dimension of the arc path all decrease. Besides, the flashover voltage of PMMA is extremely sensitive to surface charge, and the flashover voltage under extremely non-uniform electric field is less sensitive than that under the slightly non-uniform electric field. The analysis suggests that the surface charge exerts an influence on the flashover characteristics by affecting the surface field intensity and the surface discharge development process, and these factors ultimately determine the flashover characteristics.

Flashover Performance and Process of Suspension Insulator Strings Artificially Covered with Snow

Snow accumulates on the surface of insulator string, causing a decrease in its electrical performance, seriously threatening the reliable operation of the power grid. Most previous studies have focused on iced insulators; however, there is a lack of research on snow-covered insulators. In this paper, to reveal the influencing mechanism that snow has on the electrical characteristics of insulator string, based on an artificial snowing test in a chamber, the effects of equivalent salt deposit density, applied voltage type, and snow thickness on the flashover performance of snow-covered insulators are analyzed, and the flashover process is investigated. The results show that the relationship between the arc flashover gradient and the equivalent salt deposit density is a power function with a negative exponent, which is similar to that of polluted and ice-covered insulator strings. For the insulator strings with the same snow accretion, the direct current (DC) arc flashover gradient is lower than the alternating current (AC) arc flashover gradient. The relationship between arc flashover gradient and snow thickness is also a power function. The formation of a dry band during the flashover of snow-covered insulator string is similar to the flashover of the polluted insulator, and the arc propagation along the surface of the snow-covered insulator is similar to the flashover of the iced insulator.

Numerical Modelling of Ice-Covered Insulator Flashover: The Influence of Arc Velocity and Arc Propagation Criteria

This paper investigates the influence of arc velocity and propagation criteria on the parameters of a dynamic numerical mono-arc model used to predict flashover voltage of ice-covered insulators. For that purpose, a generic algorithm has been developed which, coupled with a Finite Element commercial software, permits us to solve the mono-arc Obenaus equation. The versatility of the proposed algorithm allows to implement three different arc propagation criteria and five different arc velocity criteria, as well as to compute the corresponding flashover voltage, arc velocity and leakage current. Moreover, this algorithm permits to propose a new arc velocity criterion based on numerical calculation instead of analytical formulation as proposed in literature.

Dynamic flashover model considering pollution layer resistance variation for fixed washed high voltage insulators

This paper presents a dynamic model to compute the DC flashover voltage for a fixed washed high voltage insulator. This model provides a combination between the static flashover and dynamic arc models. The proposed model takes the pollution layer resistance variation with time during washing into consideration. This variation is experimentally measured at different equivalent salt deposit density (ESDD) values. A mathematical equation for computing the pollution resistance variation with time during washing process is proposed. The DC flashover voltage is computed using the proposed flashover model at different pollution levels. The proposed flashover model is experimentally validated considering the adopted ESDD values. Also, the obtained results show a good agreement between the proposed model and experimental results. The effect of washing water flow rate on flashover voltage is preliminary investigated. Finally, the present study improves fixed washing systems and decreases the flashover during the washing process.

Effects of icing degree on ice growth characteristics and flashover performance of 220 kV composite insulators

Icing degree is one of the important factors influencing insulator's icing flashover performance. But there may be differences between the effects of icing degree under non-energized and energized icing conditions. For this purpose, the comparative tests of non-energized and energized glaze ice accumulation are conducted on 2 types of typical 220kV composite insulators under different icing degrees in this paper. Combining with the numerical calculation of electric field distribution, the monitoring of leakage current, the measurement of the ice-melting water conductivity and the observation of flashover arc paths, the influences of icing degree on ice growth characteristics and flashover voltages of composite insulators are deeply analyzed. Results indicate that with the increase of icing degree, all relevant ice parameters gradually increase but their regularities are related to the shed configuration and the position of sheds as well as whether the ice deposits on non-energized or energized insulators. These variations lead to differences of the quantity and length of icicle air gaps and the conductivity of the water film on the ice surface, and then affect the flashover arc path and icing flashover voltage. The icing flashover voltages of energized ice-covered insulators are higher than that under non-energized condition and their difference becomes larger with the increase of icing degree while the effect of shed configuration on the icing flashover performance gradually becomes less apparent when the icing becomes heavier.

Comparison of AC icing flashover performances of 220 kV composite insulators with different shed configurations

The shed configuration has obvious effects on the flow field characteristics and electric field distributions around composite insulators, and then impacts their icing flashover performance. In this study, the AC flashover comparative tests for 12 types of 220 kV composite insulators with different shed configurations under non-energized and energized ice accumulation conditions are conducted in the artificial climate chamber. The influences of shed configuration on icing flashover voltages and flashover arc paths are analyzed. Results indicate that with the increase of the icing stress, both the flashover arc path and the ratio of the air gap arc gradually decrease. These variations lead the icing flashover voltage to gradually decrease and then approach to saturation. For ice-covered composite insulators with different shed configurations, the ratios of the air gap arc to the total flashover arc are different due to the differences of their shed spacings, icicle lengths, differences of shed diameters, (extra) large shed diameters. These result in the differences of icing flashover voltages and their characteristic exponent c describing the influence of ice thickness. Besides, for insulators iced under energized condition, their icing flashover voltages and flashover arc paths are different from that under non-energized condition. Hence, the energized icing test, which is also the only normative reference of icing test proposed by the IEEE standard, should be carried out to optimize the shed configuration of composite insulators used in ice areas.

Flashover performance of ice-covered post insulators with booster sheds using experiments and partial arc modeling

The main objective of this paper is to present a new analysis, founded on partial arc activities, for evaluating the flashover performance of EHV post station insulators equipped with booster shed (BS) under heavy icing conditions. Based on observations from ordinary and high-speed cameras, novel discussions on partial arc characteristics, ice-free leakage distances, and flashover arc path were presented. Moreover, innovative axisymmetric simulations of partial arcs were implemented by using COMSOL Multiphysics™ in order to calculate their effects on potential distributions during the flashover process of the 4-, 5-, and 6-BS configurations. Simulation results of the BS configurations showed a redistribution of voltage drops along the air gaps after the appearance of the partial arc along the first air gap. This redistribution of voltage drop leads to the occurrence of partial arcs along the other air gaps which may somewhat explain occurrence of the flashover. The presented experimental and simulation studies can provide better designs for practical application of booster sheds.

Influence of shed configuration on icing characteristics and flashover performance of 220 kV composite insulators

Shed configuration is one of the important factors affecting the icing characteristics and icing flashover performance of composite insulators. In this paper, the non-energized and energized glaze icing tests are conducted on 12 types of typical 220 kV composite insulators with different shed configurations in the artificial climate chamber. Combining with numerical calculations of electrical field distributions, the monitoring of leakage current, the measurement of the ice-melting water conductivity and the shooting of flashover arc paths, the influences of shed configuration on the ice growth and icing flashover voltages of composite insulators are comprehensively analyzed. Results indicate that a larger shed diameter brings about the faster ice growth and the lower average electric field strength across the icicle air gap, which leads to the reduction of the icing flashover voltage. When the shed spacing is larger, both the icicle length and ice thickness become larger, and the ice-melting water conductivity during the flashover is higher. But the ratio of icicle air gap arc to the flashover arc is higher and its effects on the icing flashover performance are more obvious, which leads to the improvement of the icing flashover voltage. When the ice accretes on the energized insulator, the ice-melting water conductivity is smaller while the ratio of icicle air gap arc to the flashover arc is larger than that under non-energized icing condition. These variations lead to the higher icing flashover voltages for energized insulators. It is thus suggested that the icing and flashover tests under different icing degrees should be conducted on energized composite insulators to deeply study the influences of shed configuration.

Study on the DC Flashover Performance of Various Types of Insulators With Fan-Shaped Nonuniform Pollution

In areas with heavy dust or continuous wind in a single direction, fan-shaped nonuniform contamination at the windward and leeward side of the insulator string may affect flashover performance. In this paper, dc pollution flashover tests of insulators under fan-shaped nonuniform pollution were presented, and a comparison of flashover performance among 7-unit suspension insulator strings with four different disk profiles was made. Then, the influence of fan-shaped nonuniform pollution on flashover voltage, arc propagation, and critical leakage distance were investigated. Research results indicate that the insulators with different types were influenced variously on their flashover stress, utilization rate of creepage distance, and critical leakage distance by fan-shaped nonuniform pollution. A reduction in the ratio W/L of windward to leeward side salt deposit density (SDD) from 1/1 to 1/15 gave a median 35% ± 4% decrease in flashover strength. The extrapolation of results for the seven-unit strings suggests that some increment in shed space or structure height of the insulator can be preferable in dc transmission lines where there is fan-shaped nonuniform pollution accumulating at the windward and leeward side of the insulator string.

Dynamic modeling of AC multiple ARCS of EHV post station insulators covered with ice

The present contribution proposes a multi-arc dynamic model to predict the parameters of AC flashover propagating over EHV ice-cover insulators. The model considers the arc as time-dependent impedance consisting of a resistance in series with an inductance. The residual ice layer is specified in terms of an equivalent resistance, where the equivalent surface conductivity is calculated by taking into account the water film flowing over the glass surface. The temporary arc length developing over an ice-covered insulator is calculated through the arc velocity, determined using high-speed image recording techniques. The principal characteristics of flashover, including minimum flashover voltage, leakage current and arc velocity are investigated through the proposed multi-arc dynamic model. The proposed models were successfully validated in the laboratory using station post insulators - typically used in Hydro-Quebec 735 kV substations - under AC voltage based on the standard method presented in IEEE Std 1783. Good agreement was also observed between the flashover voltage calculated from the proposed multi-arc model and those obtained experimentally on ice-covered EHV insulators. The proposed model is helpful for properly designing and selecting the EHV insulators used in cold climate conditions.

Influence of the rise rate of voltage and leakage distance on flashover gradient and partial discharges level for various polymeric materials under AC stress

This paper presents results of investigations on the influence of the rise rate of voltage and the leakage distance (Lf) on flashover gradient (FOG) and partial discharges (PD) activity on insulators issued of different polymers materials in dry conditions; the aim of this study being the development of new generation of high voltage piercing connectors. The investigated polymers are of three distinct classes among the mostly used in electrical industry: thermoplastics, thermosetting and elastomers. The considered thermoplastics are polyamide 6 (PA6/50), fireproofed polyamide 66 (PA66/50), polycarbonate (PC/40) and polyarylamide (PARA/50); the thermosetting are two cycloaliphatic epoxy resins (EP1 and EP2); and as concerns elastomers, two EPDM materials noted EPDM and EPDM V0 (fireproofed) have been tested. It is observed that generally the increase of the rise rate of voltage increases the flashover gradient while the increase of leakage distance leads to the decrease of flashover gradient. The magnitude and time duration of voltage, chemical structure of polymer and leakage distance influence the magnitude of PDs. Small degradations are observed in the vicinity of high voltage electrode for thermoplastics and EPDM elastomers after the occurrence of flashover (arc) especially for PC/40. The surface of cycloaliphatic epoxy resins (EP1 and EP2) appears to be the most resistant to flashover arcs and PD activity making them suitable to be used in high voltage connectors.

Influences of grading ring arrangement on AC flashover performance of 220 KV ice-covered composite insulators

The grading ring (GR) can adjust the potential distribution along the composite insulator and reduce the electric field (E-field) strengths near its end fittings. However, little research has been conducted about the effects of GR on the insulation performance of composite insulators under icing condition. Therefore, the ac icing flashover performance of two kinds of 220 kV composite insulators under different GR arrangement patterns are tested in the artificial climate chamber. Combining with the numerical simulation, the effects are analyzed. The results indicate that when GR is installed at the high voltage (HV) end, the E-field strength close to the end fitting and the icing uniformity on the whole insulators are both improved. These cause the effect of icing degree on the icing flashover voltage to be larger than that without GR at the HV end. When GR is installed at the grounded end, the perpendicular icicles on the GR will impact the flashover arc path, which causes the icing flashover voltages to be lower than that without GR at the grounded end. When different GRs are only installed at the HV end, the characteristic exponents b (characterizing the influence of freezing water conductivity on icing flashover voltage) are all larger than that without GR. Moreover, the value of b increases with the decrease of GR’s diameter and with the raise of GR’s height. But b changes slightly when there are variations in GR’s pipe diameter and structure. The research can provide reference for the application of GR for 220 kV composite insulators in icing regions.

Modeling of AC flashover on ice-covered composite insulators with different shed configurations

The shed configuration has an obvious effect on flashover performance of ice-covered composite insulators. Based on the flashover arc path, a “multi-arc, multi-ice-layer” mathematical model is proposed to predict AC flashover voltages of ice-covered composite insulators with different shed configurations during the melting period. The model takes into account two different arc paths across the tip of each un-bridge icicle. Moreover, the arc across the air gap is distinguished from the arc on the ice surface, and the residual ice layer resistance is divided into icicle resistance, arc root resistance and shed surface ice resistance. In order to verify this model, four kinds of 220 kV composite insulators with different shed configurations are tested under different icing conditions. The theoretical results from the model are found to be in good agreement with the experimental results.

Modeling of flashover arcs in medium voltage networks due to direct lightning strikes

Abstract Distribution line faults in many cases are caused by direct lightning strokes to overhead lines resulting in a disruption between phase and ground in the form of flashover arc. Therefore, it is important to accurately simulate the lightning behaviour for the protection studies of distribution networks. This paper investigates the influence of considering the dynamic arc model and the actual volt–time characteristics of line insulators in the evaluation of lightning overvoltages. A full-scale experimental set-up was installed in the high voltage laboratory to study the dynamic interaction between fault arc and power system with grounded and ungrounded cross arm configurations. The least square method has been used to extract the arc parameters from the experimental results for modeling purpose. The model of experimental set-up has been implemented in Alternative Transients Program–Electromagnetic Transients Program (ATP–EMTP) software and the experimental results have been reproduced by computer simulations with reasonable accuracy. Based on the comparison of experimental and simulation results, it is concluded that the actual volt–time curves and the arc model that characterize the behaviour of line insulators should be considered during the simulation of the lightning performance of overhead distribution networks.

AC pollution flashover performance and flashover process of glass insulators at high altitude site

The ac pollution flashover performance and flashover process of LXHY 3 -I6O insulators was investigated at a high altitude site (altitude of 1400m) in this paper. The solid layer method was used, and even-rising voltage method was adopted. According to the test results, the average pollution flashover voltage ( U av ) decreased with the increase of pollution, and there was no obvious relationship between the exponent characterising the influence of pollution and the length of the insulator strings. The relationship between U av and the length of the insulator strings was nearly linear. Subsequently, the effect of the pollution and the length of insulator strings on the critical flashover arc trajectory were analysed. It concluded that the critical flashover distance ( l c ) decreased with the increase of pollution, and it was shorter than the leakage distance ( L 0 ), whereas there was no significant relationship between l c / L 0 and the length of insulator strings. The arc appearance at the high altitude site was very irregular with obvious bending and rocking; the local arcs deviated from the surface of the polluted insulators even more seriously and the diameter was thicker than those obtained in the normal environment. Moreover, the brightness and diameter of local arcs was related to pollution.